Why doesn't an alpha particle have a +2 charge in alpha decay?

[patrickbotros]

The following equation is written in my book:
²⁰⁴Pb→⁴α + ²⁰⁰Hg
If an alpha particle is a helium nucleus, why doesn't it have a +2 charge and why doesn't Hg have a -2 charge? Did my text just omit the charge?

 

[Borek]

Yes, in general you ignore charges when writing nuclear reactions.

Please note that once the alpha is emitted it ionizes everything around the nucleus, so we don't care about electrons, they are free to go wherever they like What is important is where the nucleons go.

In your case you start with a neutral ²⁰⁴Pb atom, but after α is emitted you don't have just ²⁰⁰Hg^2- ion, as the main nucleus recoiled, losing some of the electrons. As electrons are highly mobile they will pretty fast settle down around a new nucleus and emitted alpha, but then everything is back neutral.

 

[patrickbotros]

Yes, in general you ignore charges when writing nuclear reactions.

Please note that once the alpha is emitted it ionizes everything around the nucleus, so we don't care about electrons, they are free to go wherever they like What is important is where the nucleons go.

In your case you start with a neutral ²⁰⁴Pb atom, but after α is emitted you don't have just ²⁰⁰Hg^2- ion, as the main nucleus recoiled, losing some of the electrons. As electrons are highly mobile they will pretty fast settle down around a new nucleus and emitted alpha, but then everything is back neutral.

I don't understand what you mean. Are you saying that following the emission of an alpha particle, electrons will leave the negatively charged mercury isotope? If so, where do the electrons go to? And I'm still super unclear on how everything gets back to neutral. So I guess the main question is: what happens to the electrons that brings everything to neutral?

 

[Borek]

Electrons go everywhere, they get thrown away and - temporarily - attach to whatever they can, or just drift at random between other atoms.

After the emission you don't have just a negatively charged mercury isotope. Imagine an atom model made of of balls imitating nucleus and electrons. Let's attach those 'electrons' with strings. Hit the model with a hammer - what will happen? Most of the model 'electrons' will just get lost and they will become scattered around the room, as the strings are not strong enough to keep them up with the nucleus. That's kind of a situation you deal with during nuclear reactions. Just like the model 'electrons' can get thrown everywhere, electrons from the decaying atom are thrown away. As they are small and can easily move sooner or later they will get attracted by both new nuclei (remember, electrons are negative, nuclei are positive, so the attract each other), but before that happens they can be everywhere.

And please remember that the process (of the electrons caught back) can take a long time, while alpha particles get rather easily stopped by matter they can travel quite far - several centimeters in the air. Not only after that they are separated from their original electrons, they also ionized hundreds of atoms/molecules on their way to the place where they finally rested. There are plenty of unbound electrons around, there is a reason why alpha is called an 'ionizing radiation'.

 

[patrickbotros]

Electrons go everywhere, they get thrown away and - temporarily - attach to whatever they can, or just drift at random between other atoms.

After the emission you don't have just a negatively charged mercury isotope. Imagine an atom model made of of balls imitating nucleus and electrons. Let's attach those 'electrons' with strings. Hit the model with a hammer - what will happen? Most of the model 'electrons' will just get lost and they will become scattered around the room, as the strings are not strong enough to keep them up with the nucleus. That's kind of a situation you deal with during nuclear reactions. Just like the model 'electrons' can get thrown everywhere, electrons from the decaying atom are thrown away. As they are small and can easily move sooner or later they will get attracted by both new nuclei (remember, electrons are negative, nuclei are positive, so the attract each other), but before that happens they can be everywhere.

And please remember that the process (of the electrons caught back) can take a long time, while alpha particles get rather easily stopped by matter they can travel quite far - several centimeters in the air. Not only after that they are separated from their original electrons, they also ionized hundreds of atoms/molecules on their way to the place where they finally rested. There are plenty of unbound electrons around, there is a reason why alpha is called an 'ionizing radiation'.

Okay. I understand my original question now, but something you said sparked my curiosity. What do you mean "they also ionize hundreds of atoms/molecules on their way to the place where they finally end up". What do you mean ionize them? Like the alpha particles are so positive that they pull electrons from other atoms? Thanks for your interesting answers.

 

[Borek]

What do you mean ionize them? Like the alpha particles are so positive that they pull electrons from other atoms?

They are charged and fast, that's enough to attract electrons so strong they got removed (or bumped out, depending on how you look at it) from their original atoms.

 

[DrDu]

I just wanted to note that this effect is also used for some high voltage batteries for use, e.g. in satelites: There are two electrodes, one covered with a highly radioactive alpha source and the other electrode, separated by some mm of vacuum receives the alpha particles emitted by the first one. The first electrode charges up negatively, while the second one positively.

 

[DrDu]

See here:
http://www.google.de/url?sa=t&rct=j...TtPlz9dovnkpgdA&bvm=bv.92291466,d.bGg&cad=rja

 

[James Pelezo]

Alpha particles are emitted from unstable atomic nuclei with high neutron to proton ratios such as isotopes of Thorium, Palladium and Uranium. See link (table of radioisotopes): http://www.chemunlimited.com/Isotopes Table.pdf . The emissions only affect the nuclear composition and do not affect the electron configuration. An alpha particle is a Helium nucleus with a +2 charge upon emission from the unstable nucleus in process of becoming a stable isotope of a decay series. The equation represented in your question is a typical shorthand example representing alpha decay of Pb-204 to Hg-200. An alpha emission will reduce the atomic mass by four nucleons (2 protons & 2 neutrons) at the same time reduce the atomic number because 2 protons ( #protons = atomic number ) are removed from the nucleus of Pb-204 and changes it to Hg-200. The electronic configuration remains unchanged unless it reacts chemically with another element that will extract electrons (via oxidation process) from the valence level. In describing basic nuclear radioactivity, oxidation & reduction are not an issue. Here's a decay chain of U-238: http://www.gopixpic.com/1275/radioactive-decay-series-ppt/http:||img*docstoccdn*com|thumb|orig|59348970*png/. None of the elements in the chain change oxidation states (i.e., charge), but the atomic mass and atomic numbers are changed b/c of various modes of decay.